Eq-9 if another boat signals their intention to pass you from astern, what should you do?

It depends on which side they intend to pass on you. They should give you an audible horn blast to let you know that they want to pass you on. When passing on the starboard, they indicate one blast. For tow blasts, it indicates passing on the port. Note that you have to return the audible blast or audible horn signal in order for them to know that you have heard their signal and aware of their intentions.